Computers and other electronic devices operate primarily by conveying electrical signals between components that comprise the device. Although electrical signals may be conveyed in a number of ways, such as by radio waves and by light, the principal form of communication of electrical signals between electrical devices or components remains transmission of the signals over a conductive metallic wire. In the context of computers and digital electronics, communication is typically performed by changing the state of a wire between one of two potentials to indicate a logical 1 or 0, with transitions between states occurring at regular intervals governed by an electronic clock signal.
Ideally, when a large amount of data is to be conveyed via wire, a large number of wires can be used to provide the capacity to transmit several elements of digital data at one time. In a computer system, for example, an 8-bit byte of digital information can be transmitted in one clock cycle if eight or more communication wires are available linking the sender and receiver. As a practical matter, wires beyond the eight needed to convey the byte of data would likely be used for purposes such as sending a clock signal indicating when the data on the other wires is expected to change, sending a ground or reference signal level, and to send other information including data flow control signals.
Unfortunately, it is often impractical for reasons of cost and physical space to run a large number of conductors between every element in a circuit, or between every device in an electronic system. In many circumstances, a single pair of wires must be used to send all data between components or devices, meaning that a single pair of wires must be used for data transmission, flow control, and clock recovery in a digital system. In some such systems, one wire carries the signal while another wire is at ground or some other reference potential. In other systems, the pair of wires are driven with a differential voltage, such that the wires are driven with either the same or no voltage to represent one digital symbol or with opposite positive and negative voltages to represent a second digital symbol.
This usually requires implementation of special techniques to recover a clock signal from the digital signal transitions observed in the pair of wires. For this reason, a clock signal is sometimes sent in a third wire, making recovery of the clock unnecessary at the expense of having to supply a third conductor. The circuit designer must then decide whether this third conductor is a justifiable expense, both in terms of cost and physical routing space, and select a receiver circuit appropriate to the selected clock scheme.
It is desired to simplify the clock recovery circuit design and selection process.